Can Your Air Conditioning System Only Cool Your Space 20 Degrees?

Jul 03, 2023

We get into the “20-degree rule” and show you how to optimize your air conditioning setup.

Few experiences beat walking back into an air-conditioned house or apartment on a hot summer’s day; I should know after having lived in Tucson, Arizona, and Austin, Texas, for extended periods of time, where I’ve experienced temps as high as 110 degrees Fahrenheit. However, dropping the temperature too low could have diminishing returns. I’ve seen this supposed 20-degree rule being thrown around on Reddit, which states that AC units aren’t able to cool your space more than 20 degrees Fahrenheit below the outside temperature.

So I took it upon myself to see if this was indeed true or just another myth, and learned some valuable tips and tricks for keeping a space nice and cool in the summer months along the way.

Because I’ve been able to keep my apartments plenty cool, I was quite skeptical of this 20-degree “rule” being thrown around. Turns out, it’s more about efficiency and less about cooling capacity.

While the 20-degree rule sounds nice and neat on paper, it’s mostly insignificant in real life. “There’s no fundamental reason that an air conditioner or refrigerator can’t generate more than a 20-degree difference,” says Craig Bradshaw, director of Integrated Building Systems at Oklahoma State University. He says that the 20-degree rule is mostly just a rule of thumb used to size air conditioners properly.

Basically, all this means is that an air conditioner is designed to be at full load at a certain temperature. For instance, if it’s 95 degrees outdoors, the ideal conditions for a three-ton capacity AC unit would be 80 degrees indoors—which, according to Bradshaw, represents roughly 72-75 degree nominal conditions. If you do the math, that’s about a 20-degree difference.

“It is important, although the 20-degree rule is, I think, a bit of a myth,” says Bradshaw. I learned that the real advantage of the rule is that it helps you determine the highest temperature you should keep your thermostat set to. To offer some perspective, I generally keep my apartment at 78 degrees Fahrenheit during the day, and at about 76 degrees Fahrenheit at night, but this obviously varies from person to person.

So that’s all well and good, but the science behind how air conditioners deliver cool air is actually more interesting than you might think. “You’re not adding cooling, you’re removing heat,” says Bradshaw. Contrary to popular belief, there’s much more to air conditioning than just introducing cool air into a space. AC units are actually removing heat and humidity from the air and recirculating it.

All air conditioners pump a refrigerant fluid—that absorbs heat—through a series of coils to move heat from one location to another. One end of the system compresses the refrigerant into a liquid while the other end allows it to expand into a vapor. The aim here is to create two zones—one where the heat goes, and one where it doesn’t: the condenser creates the hot zone, which moves heat outside, while the evaporator creates the cold zone that moves cool air inside.

Your air conditioner exhausts the hot air from your space and feeds it through an outside heat exchanger, which uses a series of coils filled with refrigerant. This thermal energy has to go somewhere, and is evacuated using a fan. If you’ve ever walked outside your house and put your hand above the air conditioner, you might be surprised to feel hot air coming out. “The heat you’re feeling leaving that AC unit is all the heat that was in your house,” says Bradshaw.

Once the heat has been removed from the air, it’s reintroduced into your space as crisp, cool air. If you were paying attention in your high school chemistry class, you’ll likely remember that temperature is a measure of kinetic energy of particles within a material or system—so cold is merely the absence of heat. More energy means more heat, and vice-versa.

The Verdict

Along with just being an arbitrary number for rating cooling capacity, the 20-degree rule falls on its face when you think about those that live in hotter climates. If true, my parents down in Austin, Texas, would be in a sticky situation having dealt with triple-digit temperatures for over 40 days now; with them seeing temps as high as 110 Fahrenheit, that would mean the AC could only cool their living space down to 90 degrees. So next time somebody brings up the 20-degree rule, you can tell them it’s nothing more than hot air.

✅ How to Optimize Your Air-Conditioning Setup:→ Keep It Dark Window coverings are one of the most affordable ways to keep your place cool. Natural light is obviously good to have in any space, but too much of it can really bring the heat. This can be especially evident if you have westward- or eastward-facing windows—i.e. where the sun rises or sets. Closing the blinds at these peak hours will lower not only temperatures, but also the load on your air conditioner. → Circulate AirKeeping a number of fans going will help to circulate air throughout your living space. “This will help distribute air from areas with higher load to lower load around around the house,” says Bradshaw. Ceiling fans are great here as they move plenty of air while staying relatively quiet, but even a basic box fan will do the trick.→ Don’t PanicOne of the most interesting things I learned from Bradshaw was the fact that all AC units are sized for the environment around them—both inside and out. Too big, and it will keep clicking on and off, which is inefficient and actually rough on the air conditioner. Too small, and it simply won’t be able to get the job done. For instance, a 105-degree Fahrenheit day in Pennsylvania will likely keep my air conditioners going all day, but triple-digit temps are extremely rare.

Matt Crisara is a native Austinite who has an unbridled passion for cars and motorsports, both foreign and domestic, and as the Autos Editor for Popular Mechanics, he writes the majority of automotive coverage across digital and print. He was previously a contributing writer for Motor1 following internships at Circuit Of The Americas F1 Track and Speed City, an Austin radio broadcaster focused on the world of motor racing. He earned a bachelor’s degree from the University of Arizona School of Journalism, where he raced mountain bikes with the University Club Team. When he isn’t working, he enjoys sim-racing, FPV drones, and the great outdoors.

J-B Weld: Just How Tough Is It?

Make Your Own Smoker Out of a 55-Gallon Barrel

Simple Fixes for Toilet Trouble

These Are the 3 Most Expensive Home Reno Projects

Clean Your Bath Fan

The Right Way to Prune a Tree

Are You Sure You Know How to Use a Screwdriver?

How Do Air Purifiers Work, Exactly?

Chopping Metal is Fun With a Cold Chisel

How to Kill Those Pesky Mosquitoes in Your Yard

Watch Klein Tools Forge Its World-Famous Pliers

This Is the Best Time to Water Your Grass